Digital watermarking is a process for modifying physical or electronic media to embed a machine-readable code into the media. The media may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process. Most commonly, digital watermarking is applied to media signals such as images, audio signals, and video signals. However, it may also be applied to other types of media objects, including documents (e.g., through line, word or character shifting), software, multi-dimensional graphics models, and surface textures of objects.
Digital watermarking systems typically have two primary components: an encoder that embeds the watermark in a host media signal, and a decoder that detects and reads the embedded watermark from a signal suspected of containing a watermark (a suspect signal). The encoder embeds a watermark by altering the host media signal. The reading component analyzes a suspect signal to detect whether a watermark is present. In applications where the watermark encodes information, the reader extracts this information from the detected watermark.
Several particular watermarking techniques have been developed. The reader is presumed to be familiar with the literature in this field. Particular techniques for embedding and detecting imperceptible watermarks in media signals are detailed in the assignee's U.S. Pat. Nos. 5,862,260 and 6,614,914, which are hereby incorporated by reference.
The invention provides a method for embedding a digital watermark into a halftone image comprised of clusters of halftone dots, which are referred to as geometric primitives. In the literature on clustered-dot halftone printing, these geometric primitives are sometimes referred to as halftone dots. For color halftone images, primitives of different colors (CMYK or RGB) are combined at a pixel location to form a clustered dot having a desired color. In this document, the cluster that forms a primitive is comprised of microdots referred to as halftone dots or halftone pixel dots. The method embeds a watermark image signal comprising an array of multilevel per pixel delta values by increasing or decreasing the size of a corresponding cluster of halftone dots. In particular, each delta value represents a change in luminance in one implementation, but this delta value could correspond to other color planes of a monotone or color halftone image. The watermark embedder subtly changes the halftone primitives by toggling boundary pixels on or off in a manner that grows or shrinks the primitive.
A watermark decoder detects and reads a message in the watermarked image after it is scanned from a printed object, such as a newspaper, by detecting the changes in luminance (or other color plane) in which the watermark is embedded.
Further features will become apparent with reference to the following detailed description.